This invention relates to a power supply for an electroluminescent (EL) lamp and, in particular, to a switch circuit for connecting an EL lamp to a source of alternating current (AC).
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer includes a phosphor powder which glows in the presence of a strong electric field and a very low current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage and the current through the EL lamp ceases.
For hand held devices and other applications such as automotive displays, an EL lamp is driven by an inverter which converts direct current from a battery into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. As known to those of skill in the art, the actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the EL lamp, with frequencies below 1000 hz. being preferred. Too low of a frequency causes a noticeable flicker and low brightness. Thus, a frequency of 100-1000 hz. is preferred. Many types of inverters are commercially available for producing AC suitable for an EL lamp.
In many applications, such as automotive displays, EL lamps have a distinct advantage over other forms of lighting in the soft glow of the lamp when viewed directly. Other lamps produce a harsh glare and can only be used for indirect lighting. In addition, these other lamps are discrete (point) sources of light, requiring either a diffusing lens or a large number of small lamps to produce the desired visual effect. In either case, the cost of the display is high.
EL lamps can be manufactured in large area segments of various colors, producing striking displays. In many applications, not all segments are lit simultaneously and a suitable control circuit is needed. It is known in the prior art to drive a multi-segment display from a single inverter by individually grounding each segment through a series transistor, e.g. U.S. Pat. No. 3,908,150--Webb. In the circuit described in the Webb patent, a bipolar transistor and a lamp segment are connected in series across a source of alternating current.
A problem with this arrangement is the large DC bias that develops across a segment when the segment is off. The DC voltage is not sufficient to light the lamp but is sufficient to cause corrosion in a lamp exposed to environmental stress, e.g. temperatures considerably above room temperature (20.degree. C.). The series bipolar transistor actually blocks AC in one direction only, causing the DC bias. Using other semiconductor switches instead of a bipolar transistor is possible but is more expensive and may introduce other problems. For example, a triac could be substituted for a bipolar transistor but a triac is more difficult to turn off when in series with an EL lamp because the EL lamp is a reactive load and causes a large phase shift between voltage and current. In addition, even the least expensive triacs are more expensive than a suitable bipolar transistor.
In view of the foregoing, it is therefore an object of the invention to provide a low cost circuit for driving an EL lamp from an AC source.
Another object of the invention is to provide an interface circuit for coupling integrated circuit logic devices to an AC powered EL lamp.
A further object of the invention is to provide a switch circuit for preventing DC bias in an EL lamp driven by an AC source.